1. Field of the Invention
The present invention relates to an overcurrent protection circuit of a power semiconductor device having a current sensing electrode.
2. Description of the Background Art
FIG. 11 is a circuit diagram showing a configuration of a conventional overcurrent short-circuit protection circuit of an IGBT (Insulated Gate Bipolar Transistor) having a current sense (current sensing terminal). As shown in FIG. 11, an N-channel current sense IGBT 1 has a collector (C) connecting to a positive (+) terminal of a power supply V.sub.cc through a load LOAD, an emitter (E) connecting to a negative (-) terminal of the power supply V.sub.cc and a gate (G) terminal connecting to an output of a driver 2 through a gate resistance RG. Further, a current sensing resistance RS is connected between a sense (S) terminal and the emitter (E) terminal of the IGBT 1. The amount of the sense current flowing through the sense terminal is in proportion to that of the emitter current (=collector current I.sub.c) flowing through the emitter terminal.
The current sensing resistance RS has a node N10 on the side of the sense terminal connecting to a positive input of a comparator 3. In other words, the value of voltage drop VS across the current sensing resistance RS is fed to the positive input of the comparator 3. The comparator 3 has a reference voltage VREF1 applied to its negative input. An output of the comparator 3 is applied to an input of the driver 2. The driver 2 is thereby controlled whether to be active or non-active depending on whether the output of the comparator 3 is L-level or H-level, respectively.
The driver 2 outputs an input signal IN to the IGBT 1 when it is active while it outputs thereto an L-level signal which works to turn off the IGBT 1 when non-active.
A current detection of the IGBT 1 is conducted by the comparator 3 through comparing the voltage drop value VS across the current sensing resistance RS with the reference voltage VREF1. When the relation VS&gt;VREF1 is satisfied, the output of the comparator 3 becomes H-level to be fed back to the input of the driver 2. In consequence, the output of the driver 2 becomes L-level to turn off the IGBT 1. Accordingly, the current supply of the IGBT 1 is cut off to thereby avoid keeping the IGBT 1 in an overcurrent supply condition. Thus, the IGBT 1 is protected from falling into a latch-up condition.
The IGBT incorporating a current sense has a characteristic that the ratio of the sense current to the emitter current becomes larger when the gate voltage immediately after its turn-on is close to V.sub.th (a threshold voltage) because of feedback capacitance, as compared with when enough voltage is applied to the gate voltage.
FIG. 12 gives three timing charts showing a gate voltage V.sub.GE, collector currents I.sub.c and voltage drop values VS across the current sensing resistance RS of the IGBT incorporating a current sense at its turn-on time. In the figure, the period t immediately after its turn-on is a transition period in which the gate voltage is remaining close to the threshold value V.sub.th. During the period t, waveforms of the voltage drop values VS highly surge up, as compared with those of collector currents (I.sub.c =emitter currents). Therefore, in the case of waveform b, though the collector current I.sub.c is always equal to or less than the overcurrent level OI, the relation VS&gt;VREF1 disadvantageously holds because of the surge of the voltage drop value VS during the transition period t, to thereby cause a mistaken detection that the IGBT is in an overcurrent condition.
FIG. 13 is a circuit diagram showing a configuration of a conventional overcurrent protection circuit of the IGBT incorporating a current sense tier preventing the above mistaken detection. As shown in the figure, a capacitor CS is connected in parallel with the current sensing resistance RS. In the circuit, the current sensing resistance RS and the capacitor CS connected thereto in parallel work as a low-pass filter for preventing the mistaken detection (malfunction).
Accordingly, by setting the time constant of the filter to be sufficiently larger than the threshold state maintaining period t of the gate voltage V.sub.GE to sufficiently soften the surge of the waveform of the voltage drop value VS immediately after the turn-on of the IGBT 1, the voltage drop value VS of the IGBT 1 in normally operation can always satisfy the relation VS&lt;VREF1 even during the threshold state maintaining period t. In consequence, the IGBT 1 is not detected mistakenly being in an overcurrent condition immediately after its turn-on.
According to the configuration of FIG. 13, however, since the voltage drop value VS after being filtered is taken in through the positive input of the comparator 3, when the IGBT incorporating a current sense actually falls into an overcurrent and short-circuit condition while holding the relation VS&gt;VREF1, a time-lag for the filtering time disadvantageously occurs before breaking the IGBT, so that the IGBT can not be protected promptly and at the worst may break down.